Over the past decade or longer, the subjects of an adequate energy supply and a cleaner environment have been discussed extensively in the technical journals and newspapers of the United States. The consensus of such discussions has been that a new, major, and clean energy source is needed in our Nation, to supplement initially and to replace ultimately its declining reserves of fossil fuels.
At current and projected rates of energy consumption, the Nation's indigenous resources of petroleum may be expected to be essentially depleted early in the Twenty-First Century and its reserves of natural gas depleted not later than a decade thereafter. It is possible, even probable, that the continued use of coal as a primary fuel for production of electrical energy will, in time, fall into disuse because of economic and environmental considerations. Provision should be made now to meet such eventuality.
Of the numerous, new energy sources that have been reported publicly, little consideration has been given to extraction and use of the vast quantity of thermal energy that is known to be contained in hot, dry rock of the Earth's crust. Interest in geothermal energy has focused primarily on extraction and use of thermal energy contained in naturally occurring reservoirs of pressurized hot water or steam that exist in Northern California and at other widely scattered locations throughout the World. Generally, such naturally occurring thermal reservoirs, while important, are of relatively small capacity and uncertain life and represent only a very small fraction of accessible energy contained in hot, dry rock of the Earth's crust.
Approaches to utilization of the energy of hot, dry rock appear in U.S. Pat. Nos. 4,712,380; 4,642,987, 4,745,756; and 4,512,156. For example, U.S. Pat. No. 4,712,380 discloses heating a first working fluid by injection into a well extending down into hot, dry rock and recovery of the heated working fluid from a second adjacent well, and then using the heated working fluid to heat a more volatile second working fluid which, in turn, drives a turbine.
The total accessible quantity of such hot, dry-rock geothermal energy underlying the Continental United States has been estimated by several investigators to be between 10 million and 13 million quads. (A quad is equal to 10.sup.15 Btu, or to the chemical energy contained in 181.8 million barrels of petroleum.) If as much as 1% of such energy were extracted and used, the amount would be more than sufficient to supply the Nation's electric-energy requirements for the next 200 years, assuming a doubling of energy use every 50 years.
In addition to its value as a major energy source for utilization temperatures at or below 350.degree. C. (662.degree. F.), geothermal energy derived from hot, dry-rock within the Earth's crust, can be used in a hybrid thermal cycle, with natural gas, ethanol, or other clean-burning fuel, for large scale production of electric power and energy, with a minimal emission to the atmosphere of products of combustion. In such a hybrid cycle, sensible and latent heat required to produce dry, saturated steam at a relatively high pressure, would be supplied from a geothermal source. The dry, saturated steam would then be superheated, to a desired temperature, by heat derived from combustion of a fossil fuel, natural gas, or a clean burning chemical fuel. For steam conditions of 1800 pounds per square inch pressure and 1000.degree. F. temperature, 78% of the required heat energy would be supplied from a geothermal source and 22% from a fossil fuel source.
The hybrid cycle would yield two other important benefits:
(1) It would assure an exact match between turbine design temperature and actual temperature of available geothermally-produced steam, and PA1 (2) It would provide a means to correct a reasonable decline in steam temperature, over passage of time, if the geothermal energy source were not fully renewable.